Power plants adjust the supply of electric power from power generators in accordance with ever-changing electric power demand. The phase “changing electric power demand” as used herein includes not only fluctuations in the amount of electric power consumed by a consumer, but also fluctuations in the amount of natural energy generation such as wind power generation or photovoltaic power generation in power plants built by a consumer or a power generation dealer, and fluctuations due to accidental disconnection.
Frequencies of electric power supplied from power generators are characterized in that they decrease with increasing demand and increase with decreasing demand. However, a significant deviation of the power frequencies from a prescribed value may adversely affect consumer's equipment, and yet it is expected that a more significant deviation may have an influence on power generation facilities, and in the worst case, may cause a power failure. In view of this, the power plants manage the supply and demand of electric power by balancing it so that the power frequencies are maintained at prescribed values. Normally, when a frequency drift has been detected, a power generator or a power generator control apparatus performs control for returning the frequency to a prescribed value by adjusting the amount of fuel input to the power generator in accordance with the frequency drift so as to adjust the supply of electric power.
However, since such control is generally performed after a delay and is thus too late when the rate of change in supply-and-demand balance exceeds a threshold value, the frequency drift continues to occur for a while. Note that the threshold value varies depending on the operating conditions of power generators (a total of rated outputs of power generators that are in operation). For example, the threshold value increases with increasing total value of the rated output values of power generators in operation, thus allowing the power generators to follow a great rate of change.
While the electric power supplied from the power generators has characteristics as described above, recent progress in the introduction of power generators that convert natural energy having a threshold value liable to variation into electric energy will be considered to lead to an increase in the possibility of occurrence of phenomena in which frequencies are unstable as described above. In view of this, many techniques such as disclosed in Patent Document 1 are being considered, in which frequencies are stabilized by additionally providing a storage battery facility (hereinafter, simply referred to as a “storage battery”) that includes a capacitor and an inverter board and causing the output of the storage battery to follow demand fluctuations that are unable to be appropriately followed by power generators. Such a system stabilization technique is being considered for use not only in isolated islands but also in large-scale electric power systems.